For safety purposes, an increasing number of motor vehicles have been made to possess monitoring systems with sensors mounted on each of the tire and wheel assemblies of the vehicle. The monitoring systems are dedicated to measuring one or more parameters, such as pressure and/or temperature of the tires, and are intended to inform the vehicle's driver of any abnormal variation in the measured parameter(s).
These monitoring systems customarily include a sensor placed in each tire and wheel assembly with a microprocessor and a radiofrequency or RF emitter, as well as a central unit for receiving signals emitted by the RF emitters, the central unit including a computer integrating a radiofrequency receiver connected to an antenna.
One of the problems presented by such conventional monitoring systems resides in the need to associate, with each signal received by the receiver of the central unit, information relating to the location of the sensor and therefore of the tire and wheel assembly from which this signal originates. This need persists for the lifetime of the vehicle, that is, this need remains even after changes of tire and wheel assemblies and even after simple changes in the positioning of these assemblies on the vehicle.
European patent document EP 1 669 221 A1 presents a device configured to be carried by a tire and wheel assembly so as to locate a right or left position of the assembly in a vehicle. The device includes:                two magnetic sensors with axes of maximum sensitivity;        a measurement circuit for measuring a signal from each of the magnetic sensors, and for delivering two periodic signals phase-shifted with respect to each other, with each periodic signal being representative of variations in magnetic field values detected by the magnetic sensors during a revolution of the tire and wheel assembly; and        a calculation processor programmed to determine, on the basis of the phase shift between the two periodic signals, a direction of rotation of the tire and wheel assembly, and to deduce, from this direction of rotation and a direction of travel of the vehicle, the right or left location of the tire and wheel assembly.        
This device is such that the axes of maximum sensitivity of the two magnetic sensors are configured to be placed in the tire and wheel assembly in a secant plane with respect to a rotation axis of the tire and wheel assembly, and offset with respect to one other, in this secant plane, by a predetermined angle.
The two sensors of this device detect a global magnetic field that surrounds them, composed of a terrestrial magnetic field plus a vehicle environmental magnetic field. This vehicle environmental magnetic field is the resultant of a set of vehicle fields created by the presence of electrical or magnetic equipment on board the vehicle as well as by metallic parts close to the vehicle's wheel arches, such as brake calipers and suspension elements, for example.
However, there exist points on the terrestrial globe and directions of travel of the vehicle for which the contribution of the terrestrial magnetic field measured by the two sensors of the above device becomes negligible in relation to the contribution of the environmental magnetic field, and this may pose problems in utilizing the signals from the two sensors.